1. Field of the Invention
The present invention relates to a heat-sensitive recording material which has an excellent anti-frictional charge property as well as an excellent water resistance and an ability to suppress image-color degradation.
2. Description of the Related Art
A heat-sensitive recording material is a recording material having a structure in which a heat-sensitive coloring layer (hereinafter, also called as ‘heat-sensitive recording layer’), where a color is developed by heating, is formed on a substrate such as paper, synthetic paper, and resin film. A thermal printer having a built-in thermal head is used for heating, for the color development.
A method for recording on the heat-sensitive recording material has advantages that it is able to record in a short time by using a comparatively simple apparatus without having a need to perform processes such as developing and fixing as well as its low cost, as compared with other methods for recording. The method for recording on the heat-sensitive recording material is used in many fields: POS field such as perishable foods, boxed meals, and prepared food; copying field such as books and documents; communication field such as facsimile; ticketing field such as ticket vending machines and receipts; and aviation industry such as baggage tags. Among these fields, the method for recording on the heat-sensitive recording material has been used at a rapid pace particularly in POS field for products such as boxed meals and prepared food where hot food is a main selling point as well as for products such as ham which is served cold, and perishable food which is prone to get wet at a low temperature.
In such heat-sensitive recording material, from a view point of properties as recording paper such as dimensional stability, physical strength, and insolubility in water, synthetic paper and plastic film are largely used as a substrate. Particularly, film substrates such as synthetic paper are largely used for applications such as labels for food which is served cold, and baggage tags, and special applications such as labels for sticking on test tubes and beakers used in research laboratories.
However, a film substrate such as synthetic paper has an higher electrical resistance as compared to that of paper, and therefore, static electricity tend to be produced easily due to the friction with components such as a platen roll and a thermal head while running the synthetic paper in a printer during printing. As a result, defects during running such as jamming of the paper and a trouble such as damage of the thermal head tend to occur easily.
Such static electricity is produced mainly by a frictional charge between the platen roll and the heat-sensitive recording material. Therefore, measures such as applying an antistatic agent on a rear surface of the heat-sensitive recording material, which comes in contact with the platen roll, have been taken. Examples of such antistatic agent include: (1) inorganic salts such as sodium chloride, (2) anionic polyelectrolytes such as sodium polystyrenesulfonate, and (3) conductive metallic compounds such as conductive zinc oxide and tin oxide.
However, the inorganic salts such as sodium chloride in (1) and the anionic polyelectrolytes such as sodium polystyrenesulfonate in (2) have a low antistatic effect corresponding to the used amount. In addition, these agents have shortcomings such as being sticky in high humidity and easily dissolved in water.
Moreover, regarding the conductive metallic compounds in (3), although they do not have temperature dependence and show an effect with a minute amount, they have a drawback in terms of chemical safety being used as a heat-sensitive recording material. Particularly, in the fields of POS labels, tags and CAD, various properties such as physical strength against bending and tearing, dimensional stability, and insolubility in water are sought to be satisfied at the same time because of the object of application. However, the above-mentioned compounds are not capable of satisfying sufficiently all these properties.
As a means to solve these problems, heat-sensitive films in which delivery problems of feeding multiple sheets due to static electricity and paper jamming are solved by using a styrene copolymer of an aliphatic quaternary salt in an under layer and a back layer, and which have strong paper-quality strength are proposed (Japanese Patent Application Laid-Open (JP-A) Nos. 62-032080, 01-097679 and 01-291981).
Moreover, JP-A Nos. 06-234270, 2000-263935 and 2002-248864 propose heat-sensitive recording materials in which the trouble during paper delivery and the stickiness are dealt with by providing at least one of the back layer and an under layer which includes one of a polymer of a quaternary salt and an acrylic ester copolymer, and a polymer of a quaternary salt, a water soluble resin, and a water resisting agent as a main component, and further which has excellent water resistance are proposed.
However, in these related arts, a quaternary salt styrene based polymer (a block copolymer of styrene having an aliphatic quaternary ammonium group with a styrene monomer) is used as a quaternary salt polymer. This quaternary salt styrene based polymer has a shortcoming that the density of a print image is degraded during preservation (particularly under high temperature conditions) after printing. The mechanism of this image color degradation has not been clearly known, but it is considered that the quaternary salt styrene based copolymer which is used in the under layer is scattered up to a heat-sensitive coloring layer in a high-temperature environment with easy mass-transfer conditions where a bonding of a leuco dye and a developer is inhibited in some way.
Moreover, such type of color degradation is observed even when the quaternary salt styrene based copolymer is used in the back layer. For example, when the back layer and a front layer (protective layer) come in contact in a rolled form and when samples after printing are stored in piles, the quaternary salt styrene based copolymer comprised in the back layer scatters to the front layer (protective layer) with which the quaternary salt styrene based copolymer is in contact and gives rise to a similar phenomenon of color degradation.
To deal with such image color degradation, the quaternary salt styrene based copolymer, as described in JP-A No. 02-155688, was considered to be a material with which the color degradation cannot occur easily than with other antistatic agents. However, when preservation in a severe environmental conditions such as high temperature conditions and high humidity conditions is taken into consideration, there is still a lacking factor and a further improvement and development is still expected to be done in the present situation.